Electrochemical system for metal recovery

ABSTRACT

Metal recovery apparatus ( 20 ) being a diaphragm-less electrolytic cell arrangement where oxidizing reactions are achieved in a cathode cell ( 32 ) in an anode mode and reducing reactions in the anode cell ( 43 ) in a cathode mode by changing the direction of current flow. Better performance is achieved by pulsing ( 27   b ) the DC power ( 27   b ) to the electrodes and in some applications, installing plastic mesh ( 165 ) over the surface of the solution electrodes ( 24, 31 ) to reduce unwanted side reaction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage Application claiming the priority of co-pending PCT Application No. PCT/AU2007/001102 filed Aug. 6, 2007. Applicant claims the benefits of both 35 U.S.C. §119 and 35 U.S.C. §120 as to the PCT application, and the entire disclosure of said application is incorporated herein by reference in its entirety.

FIELD OF INVENTION

This invention relates to the electrolytic extraction of metals from their compounds in a more efficient commercial process.

INTRODUCTION

A diaphragm-less electrolytic system for the recovery of metals is disclosed in U.S. Pat. No. 5,882,502. This device has some commercial shortcomings and the present invention is directed to improvements therein.

DISCLOSURE OF THE INVENTION

The invention is said to reside in an electrochemical metal recovery apparatus comprising a first cell and a second cell, the first cell including a first main electrode and a first solution electrode and the second cell including an second main electrode and a second solution electrode, a power supply to supply DC power to the first and second cells, an electrical connection between the main electrode of one of the cells with the solution electrode of the other of the cells via the power supply and a direct electrical connection between the main electrode of the other of the cells with the solution electrode of the other of the cells, an electrolyte supply to the first cell, an enriched electrolyte transfer line between the first cell and the second cell and a depleted electrolyte line from the second cell, whereby metal is taken into solution from a metal source in the first cell and is deposited on the electrodes in the second cell.

In one embodiment the first cell is an anode cell and the second cell is a cathode cell.

In an alternative the first cell is a cathode cell and the second cell is an anode cell.

In a further embodiment the first cell comprises a first cell assembly of an anode cell and a cathode cell and the second cell comprises a second cell assembly of an anode cell and a cathode cell.

There can be further included a frequency modulator associated with the DC power source whereby to supply a pulsed DC power to the first and second cells. The frequency modulator can have a frequency range of from 10 to 100 kilohertz and a duty cycle of the pulsing DC power of from 10 to 90 percent.

The first main electrode and the second main electrode may be made of expanded titanium sheet coated with platinum group oxides. Alternatively the first main electrode and the second main electrode may be made from expanded metal sheets selected from aluminum, iron, copper or stainless steel.

Preferably the first main electrode comprises an expanded metal sheet and is sandwiched between two first solution electrodes and the second main electrode comprises an expanded metal sheet and is sandwiched between two second solution electrodes and non-conductor baffles are installed between the respective main electrodes and the solution electrodes whereby to force electrolyte flowing through the respective cells to weave in and out of the expanded metal electrodes.

Preferably the solution electrodes are plain sheet metal.

There can be further included a plastic mesh covering the surface of each of the solution electrodes whereby to trap a layer of electrolyte thereagainst.

The metal source can be scrap metal, metal ores or metal ore concentrates. For instance the metal may be copper such as from copper scrap metal, copper ore or copper concentrate which is placed into the anode cell.

In an alternative form the invention resides in an electrochemical metal recovery apparatus comprising a first electrochemical cell assembly and a second electrochemical cell assembly, the first electrochemical cell assembly comprising a first anode cell and a second anode cell, the first anode cell including a first main electrode and a first solution electrode, the second anode cell including a second main electrode and a second solution electrode, a first power supply to supply DC power to the first and second anode cells, an electrical connection between the first main electrode of the first anode cell with the second solution electrode of the second anode cell via the first power supply and a direct electrical connection between the second main electrode of the second anode cell with the first solution electrode of the first anode cell, the second electrochemical cell assembly comprising a first cathode cell and a second cathode cell, the first cathode cell including a third main electrode and a third solution electrode, the second cathode cell including a fourth main electrode and a fourth solution electrode, a second power supply to supply DC power to the first and second cathode cells, an electrical connection between the third main electrode of the first cathode cell with the fourth solution electrode of the second cathode cell via the second power supply and a direct electrical connection between the fourth main electrode of the second cathode cell with the third solution electrode of the first cathode cell, an electrolyte supply to the first anode cell, a first electrolyte transfer line between the first anode cell and the second anode cell, an enriched electrolyte transfer line between the second anode cell and the first cathode cell, a second electrolyte transfer line between the first cathode cell and the second cathode cell and a depleted electrolyte line from the second cell, whereby metal is taken into solution from a metal source in the first cell assembly and is deposited on the electrodes in the second cell assembly.

This system provides close control of the voltage for dissolution of the metals in the first assembly and close control of the voltage in the deposition of the metals in the second assembly.

Preferably each of the first power supply and the second power supply comprise a frequency modulator associated with the DC power source whereby to supply a pulsed DC power to the first and second cell assemblies, each of the frequency modulators have a duty cycle of the pulsing DC power of from 10 to 90 percent and each of the frequency modulators have a frequency range of from 10 to 100 kilohertz. Each of the power supplies can be operated at the optimum voltage required for the dissolution of the metal in the first assembly and the deposition of the metal in the second assembly.

There may be more than one assembly to dissolve the metals and there may be more than one assembly to deposit the same or different metals.

There may be a purification of the electrolyte between the first assembly and the second assembly including solvent extraction, pH control, or cementation.

Preferably each of the main electrodes can be made of expanded titanium sheet coated with platinum group oxides. Alternatively each of the main electrodes can be made from expanded metal sheets selected from aluminum, iron, copper or stainless steel.

Preferably each of the solution electrodes are plain sheet metal.

Preferably the each of the main electrode comprises an expanded metal sheet and is sandwiched between respective solution electrodes and non-conductor baffles are installed between the respective main electrodes and the solution electrodes whereby to force electrolyte flowing through the respective cells to weave in and out of the expanded metal electrodes.

There can be further including a plastic mesh covering the surface of each of the solution electrodes whereby to trap a layer of electrolyte thereagainst.

The metal source can be scrap metal, metal ores or metal ore concentrates. For instance the metal may be copper such as from copper scrap metal, copper ore or copper concentrate which is placed into the anode cell assembly.

This then generally describes the invention but to assist with understanding reference will now be made to the accompanying drawings which show preferred embodiment of the invention.

In the drawings:

FIG. 1 shows a first embodiment of an electrochemical metal recovery cell according to the present invention;

FIG. 1A shows a graph of pH vs time for the operation of the embodiment shown in FIG. 1;

FIG. 2 shows a second embodiment of an electrochemical metal recovery cell according to the present invention in cathode mode;

FIG. 2A shows a graph of pH vs time for the operation of the embodiment shown in FIG. 2;

FIG. 3 shows a graph of the pulsing frequency versus the hydrogen peroxide generation for the embodiment shown in FIG. 1;

FIG. 4 shows detail of the construction of the electrodes according to a preferred embodiment of the invention; and

FIG. 5 shows an alternative embodiment of an electrochemical metal recovery apparatus according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows one embodiment of a metal recovery apparatus of the present invention. The metal recovery apparatus 20 has an anode cell 23 and a cathode cell 32. The anode cell has an anode 25 and an anode solution electrode 24. The cathode cell has a cathode 30 and a cathode solution electrode 31. A power supply 27 is connected between the anode 25 and the cathode solution electrode 31. A direct electrical connection 26 is provided between the cathode 30 and the anode solution electrode 24. Depleted electrolyte is provided to the anode cell at 28 and metal from a source (not shown) is taken into solution in the anode call. An enriched electrolyte line 22 transfers enriched electrolyte from the anode call 23 to the cathode cell 32. In the cathode cell metal is deposited from solution. Depleted electrolyte is removed from the cathode cell 32 by line 29 and can be recycled to the anode cell.

The power supply 27 comprises a power source 27 a which can supply variable voltage and variable current and a frequency modulator 27 b which can supply variable frequency and a variable duty cycle.

FIG. 1 shows a system in which the cathode electrode 30 is acting as an anode for the maximum metal recovery. The change is achieved mainly by interchanging the connections to the cathode electrode 30 and the cathode solution electrode 31. The flow of electrons is reversed so that electrons are removed from the catholyte 29 by the cathode electrode in the same way that electrons are removed from the anolyte solution 28 by the anode electrode 25.

The metal to be recovered may be copper such as from copper scrap metal, copper ore or copper concentrate which is placed into the anode cell.

FIG. 1A shows a graph of the pH of the anolyte and catholyte produced by the arrangement of FIG. 1 and shows that the pH of both anolyte and catholyte are raised initially before falling to below 4.0 after 120 minutes. This is consistent with both the catholyte and anolyte both being acidic indicating oxidizing reactions.

FIG. 2 shows one embodiment of a metal recovery apparatus of the present invention. The metal recovery apparatus 40 has an anode cell 43 and a cathode cell 52. The anode cell has an anode 45 and an anode solution electrode 44. The cathode cell has a cathode 50 and a cathode solution electrode 51. A power supply 47 is connected between the cathode 50 and the anode solution electrode 44. A direct electrical connection 46 is provided between the anode 45 and the cathode solution electrode 51. Enriched electrolyte is provided to the anode cell at 48 and metal is deposited in the anode call. A depleted electrolyte line 42 transfers depleted electrolyte from the anode call 43 to the cathode cell 52. In the cathode cell metal is taken into solution from a metal source (not shown). Enriched electrolyte is removed from the cathode cell 52 by line 49.

The power supply 47 comprises a power source 47 a which can supply variable voltage and variable current and a frequency modulator 47 b which can supply variable frequency and a variable duty cycle.

FIG. 2 shows a system in which the anode electrode 45 is acting as a cathode electrode. In this cathode mode, both anode electrode 45 and cathode electrode 50 are adding electrons to the anolyte 48 and catholyte 49. In this mode, reducing reactions are produced.

FIG. 2A shows a graph of the pH of the anolyte and catholyte produced by the arrangement of FIG. 4 and shows that both anolyte 48 and catholyte 49 show an increase of pH over time.

FIG. 3 shows a graph of pulsing frequency of a DC voltage versus the hydrogen peroxide produced for the embodiment shown in FIG. 1. The graph was taken based on constant voltage and at atmospheric pressure. The graph shows an increase in hydrogen peroxide production as the frequency is increased up to 50 kilohertz.

FIG. 4 shows detail of the construction of the electrodes according to a preferred embodiment of the invention. In this embodiment, which is applicable to both the anode cell assembly and the cathode cell assembly 158, the electrode (cathode or anode) 160 is formed from an expanded metal sheet to give it a large surface area, active sites and to encourage turbulent flow over the surface of the electrode. The electrode may be formed from iron, aluminum or stainless steel (316 stainless steel) with or without a coating to prevent corrosion and to provide a low over-voltage. Alternatively the electrode may be titanium coated with platinum group oxides. Around the electrode 160 is a baffle arrangement 162. The baffle arrangement 162 is formed from an electrically non-conductive material and is placed to force the electrolyte to weave in and out of the expanded metal electrode. Surrounding the baffle arrangement are sheet metal solution electrodes 164. The solution electrodes may be constructed from titanium coated with platinum group oxides or stainless steel (316 stainless steel). On each side of the solution electrodes is a plasctics material mesh 165 which holds a proportion of the electrolyte stagnant around the solution electrodes. The still enables electronic transfer to and from the solution electrodes as required but prevents ionic reactions. Electrolyte flow through the electrode assembly is shown by the dotted line 166. It will be seen that the electrolyte follows a tortuous path thereby encouraging good contact with the respective electrode.

FIG. 5 shows an alternative embodiment of an electrochemical metal recovery apparatus according to the present invention. In this embodiment there is an anode cell assembly 100 and a cathode cell assembly 102. The anode cell assembly comprises a first anode cell 104 and a second anode cell 106. The first anode cell has a first main electrode 108 and a first solution electrode 110. The second anode cell has a second main electrode 112 and a second solution electrode 114. A first power supply 116 is connected between the first main electrode 108 and the second solution electrode 114. A direct electrical connection 118 is provided between the second main electrode 112 and the first solution electrode 110. Depleted electrolyte is provided to the anode cell assembly at 120 and metal taken into solution in both the first anode cell 104 and the second anode cell 106. An enriched electrolyte line 122 transfers enriched electrolyte from the first anode cell 104 to the second anode cell 106. The metal to be recovered may be copper such as from copper scrap metal, copper ore or copper concentrate which is placed into the first anode cell 104 and the second anode cell 106.

An enriched electrolyte line 126 extends to the anode cell assembly 102 via a purification stage 128. In the purification stage 128 the enriched electrolyte can be filtered or otherwise purified to remove contaminants which may affect purity of deposited metal at the cathode stage.

The cathode cell assembly comprises a first cathode cell 134 and a second cathode cell 136. The first cathode cell has a third main electrode 138 and a third solution electrode 140. The second cathode cell 136 has a fourth main electrode 142 and a fourth solution electrode 144. A second power supply 146 is connected between the fourth main electrode 142 and the third solution electrode 140. A direct electrical connection 148 is provided between the third main electrode 138 and the fourth solution electrode 144. Enriched electrolyte is provided to the cathode cell assembly at 130 after purification and metal deposited in both the first cathode cell 134 and the second cathode cell 136. A partially depleted electrolyte line 150 transfers partially depleted electrolyte from the first cathode cell 134 to the second cathode cell 136. The metal recovered may be copper. A depleted electrolyte line 152 transfers depleted electrolyte back to the anode cell assembly 100.

Each of the power supplies 116 and 146 comprises a power source which can supply variable voltage and variable current and a frequency modulator which can supply variable frequency and a variable duty cycle.

An important commercial benefit of the embodiment shown on FIG. 5 is that there is a separate DC power source for the anode electrodes and the cathode electrodes. There is an optimum voltage for dissolving compounds in the anode cell assembly and an optimum voltage for precipitating metals in the cathode cell assembly.

Experiments and Discussion

Experiments were carried out using a large scale laboratory electrolytic apparatus fitted with 50 mm×500 mm titanium electrodes, operated in unipolar mode, meaning the liquid going through the anode cell is separate from the liquid going through the cathode cell. In the unipolar mode, the anolyte water became acidic and the catholyte water became alkaline. It was discovered that by interchanging the connections to the cathode electrode and the cathode solution electrode as shown on FIG. 1, the cathode behaved like an anode, that is oxidizing reactions occurred instead of reducing reactions. Interchanging the connections meant that electrons are being removed from the cathode electrode, instead of being added to the cathode electrode. The direction of the current to the cathode electrode is the same as the direction of the current at the anode. When the pH of the anolyte and the catholyte was measured, the catholyte water became acidic similar to the anolyte water as shown on FIG. 1A. Similarly when the connections to the anode and the anode solution electrodes were interchanged as shown on FIG. 1, the anode behaved like a cathode. FIG. 2A shows the anode water and the cathode water became alkaline when the anode was connected like a cathode.

This discovery has important technical and commercial discovery. Measurements have shown that the total voltage is the sum of the voltage between the anode and the anode solution electrode and between the cathode and the cathode solution electrode. Further, the voltage between the anode electrode and the anode solution electrode was proportional to the gap between these electrodes. This is the same for the cathode. The voltage between the gaps was affected also by the resistance of the electrolyte and the current density.

Another important application is in extracting metals from a waste process stream where the cells can be operated in cathode mode. Some chemical processes require strong oxidizing conditions and the cells can be operated in the anode mode.

Another important discovery was that pulsing the current to the electrolytic cells resulted in a higher performance of the electrolytic system in terms of the quantity of reaction as measured by the amount of product produced. The frequency unit that can vary the frequency of pulsing and the duty cycle is shown in FIGS. 1 and 2. The largest pulsing unit I have built for experiments on water treatment is a 40 volt×750 amperes output with frequency from 20 kilohertz to 100 kilohertz. FIG. 3 shows a graph showing the production of hydrogen peroxide increasing with the frequency of the pulsing at a constant voltage and duty cycle. A 50% duty cycle means the current is applied 50% of the time and the rest of the time there is no current flowing. A square wave is found to be the most efficient.

Another important discovery was the construction of the electrodes for efficient electrochemical reactions. Firstly, the coating that is found effective is made from oxides of the platinum group metals. The best physical construction for the anode and cathode electrode is the expanded metal electrode and the solution electrodes are made of plain sheets. As shown on FIG. 4, the anode or cathode electrodes were sandwiched between the plain solution electrodes and non-conductor baffles are installed to force the electrolyte to weave in and out through the expanded metal electrodes. This reduces polarization aside from the large number of active sites in the expanded metal electrodes.

The anode electrode and the cathode electrode may be made of expanded titanium metal sheet and coated with platinum group metals if the application requires these electrodes not to participate in the electrolysis reaction. Cheaper materials may be considered in some applications such as antimonial lead sheets. Where the application calls for the anode and cathode electrodes to participate in a reaction, the materials may include aluminum, iron, zinc, and copper.

The last important discovery is covering the surface of the solution electrodes with a plastic mesh to maintain a stagnant layer of liquid to ensure that the anode or cathode solution electrodes did not participate in the reaction. This will assure that in electrolytic reactions where ions in solution participate in the reaction such as the ferrous-ferric ion or the cuprous-cupric ions were not reacted at the surface of the solution electrodes as this would reduce the current efficiency of the reaction. One effect of the non-conductor plastic mesh is to increase the current density at the solution electrodes. Testing with or without the plastic mesh cover for a particular application will determine if the plastic cover is necessary for that application. 

1. An electrochemical apparatus comprising a first cell and a second cell, the first cell including a first main electrode and a first solution electrode and the second cell including an second main electrode and a second solution electrode, a power supply to supply DC power to the first and second cells, an electrical connection between the main electrode of one of the cells with the solution electrode of the other of the cells via the power supply and a direct electrical connection between the main electrode of the other of the cells with the solution electrode of the other of the cells, an electrolyte supply to the first cell, an enriched electrolyte transfer line between the first cell and the second cell and a depleted electrolyte line from the second cell, whereby metal is taken into solution from a metal source in the first cell and is deposited in the second cell.
 2. An electrochemical apparatus as in claim 1 wherein the first cell is an anode cell and the second cell is a cathode cell.
 3. An electrochemical apparatus as in claim 1 wherein the first cell is a cathode cell and the second cell is an anode cell.
 4. An electrochemical apparatus as in claim 1 wherein the first cell comprises a first cell assembly of an anode cell and a cathode cell and the second cell comprises a second cell assembly of an anode cell and a cathode cell.
 5. An electrochemical apparatus as in claim 1 further including a frequency modulator associated with the DC power source whereby to supply a pulsed DC power to the first and second cells.
 6. An electrochemical apparatus as in claim 6 wherein the frequency modulator has a frequency range of from 10 to 100 kilohertz.
 7. An electrochemical apparatus as in claim 6 wherein the frequency modulator has a duty cycle of the pulsing DC power of from 10 to 90 percent.
 8. An electrochemical apparatus as in claim 1 wherein the first main electrode and the second main electrode are made of expanded titanium sheet coated with platinum group oxides.
 9. An electrochemical apparatus as in claim 1 wherein the first main electrode and the second main electrode are made from expanded metal sheets selected from aluminum, iron, copper or stainless steel.
 10. An electrochemical apparatus as in claim 1 wherein the first main electrode comprises an expanded metal sheet and is sandwiched between two first solution electrodes and the second main electrode comprises an expanded metal sheet and is sandwiched between two second solution electrodes and non-conductor baffles are installed between the respective main electrodes and the solution electrodes whereby to force electrolyte flowing through the respective cells to weave in and out of the expanded metal electrodes.
 11. An electrochemical apparatus as in claim 1 further including a plastic mesh covering the surface of each of the solution electrodes whereby to trap a layer of electrolyte thereagainst.
 12. An electrochemical apparatus comprising a first electrochemical cell assembly and a second electrochemical cell assembly, the first electrochemical cell assembly including comprising a first anode cell and a second anode cell, the first anode cell including a first main electrode and a first solution electrode, the second anode cell including a second main electrode and a second solution electrode, a first power supply to supply DC power to the first and second anode cells, an electrical connection between the first main electrode of the first anode cell with the second solution electrode of the second anode cell via the first power supply and a direct electrical connection between the second main electrode of the second anode cell with the first solution electrode of the first anode cell, the second electrochemical cell assembly including comprising a first cathode cell and a second cathode cell, the first cathode cell including a third main electrode and a third solution electrode, the second cathode cell including a fourth main electrode and a fourth solution electrode, a second power supply to supply DC power to the first and second cathode cells, an electrical connection between the third main electrode of the first cathode cell with the fourth solution electrode of the second cathode cell via the second power supply and a direct electrical connection between the fourth main electrode of the second cathode cell with the third solution electrode of the first cathode cell, an electrolyte supply to the first anode cell, a first electrolyte transfer line between the first anode cell and the second anode cell, an enriched electrolyte transfer line between the second anode cell and the first cathode cell, a second electrolyte transfer line between the first cathode cell and the second cathode cell and a depleted electrolyte line from the second cell, whereby metal is taken into solution from a metal source in the first cell and is deposited in the second cell.
 13. An electrochemical apparatus as in claim 12 wherein each of the first power supply and the second power supply comprise a frequency modulator associated with the DC power source whereby to supply a pulsed DC power to the first and second cell assemblies.
 14. An electrochemical apparatus as in claim 14 wherein each of the frequency modulators have a duty cycle of the pulsing DC power of from 10 to 90 percent.
 15. An electrochemical apparatus as in claim 14 wherein each of the frequency modulators have a frequency range of from 10 to 100 kilohertz.
 16. An electrochemical apparatus as in claim 12 wherein each of the main electrodes are made of expanded titanium sheet coated with platinum group oxides.
 17. An electrochemical apparatus as in claim 12 wherein each of the main electrodes are made from expanded metal sheets selected from aluminum, iron, copper or stainless steel.
 18. An electrochemical apparatus as in claim 1 wherein the each of the main electrode comprises an expanded metal sheet and is sandwiched between respective solution electrodes and non-conductor baffles are installed between the respective main electrodes and the solution electrodes whereby to force electrolyte flowing through the respective cells to weave in and out of the expanded metal electrodes.
 19. An electrochemical apparatus as in claim 12 further including a plastic mesh covering the surface of each of the solution electrodes whereby to trap a layer of electrolyte thereagainst. 